The present invention relates to a solid state laser which emits laser light by dint of the energy of excitation light incident to one side of its laser element.
FIGS. 10A and 10B illustrate in combination a side excitation type solid state laser which is excited by semiconductor laser light incident to one side of the laser element, FIG. 10A being a plan view and FIG. 10B its cross-sectional view. Reference numeral 7 denotes a semiconductor laser for excitation use, 8 a solid state laser element, 9 a concave mirror which has a reflectivity of 99% or more with respect to the wavelength of the solid state laser, and 10 a plane mirror which has a transmittivity of 10% or so. The concave mirror 9 and the plane mirror 10 constitute a resonator. Reference numeral 11 indicates a reflecting mirror whereby excitation light having passed through the laser element is reflected back thereinto, the hatched part indicating an area where the excitation light is absorbed. Reference numeral 6 denotes a laser beam.
The side excitation type solid state laser has its feature in that the output can be increased simply by increasing the number of semiconductor lasers for excitation use, without the necessity of using any special optical systems, and that the energy of excitation light is absorbed in the direction of the optical axis of the laser element. Hence the side excitation type solid state laser is considerably low in the non-uniformity of a temperature rise like an end face excitation type solid state laser and suitable for use as a high output solid state laser.
The side excitation type solid state laser has a defect of low excitation efficiency, especially in a single fundamental transverse mode of operation.
That is, since the conventional solid state laser element has its laser medium uniformly added thereto, the excitation light incident to the laser element is also absorbed in other regions than the laser region as shown in FIG. 10B. The energy of the excitation light which contributes to the laser oscillation is a part of the total energy of the incident excitation light and the ratio of the energy of the excitation light for laser oscillation to the total energy of the incident excitation light is particularly low, resulting in the excitation efficiency being impaired.
Moreover, since the reflecting mirror attached to the conventional laser element usually covers its side only up to about 50% of the total side surface area, the incident excitation light is reflected back and forth only once in the laser element and the excitation efficienty cannot radically be improved.
Besides, the conventional side excitation type solid state laser using semiconductor lasers has a shortcoming that the laser element containing a laser medium of a low absorption coefficient is very difficult of oscillation.